The need to transport high-bandwidth signals from place to place continues to drive growth in the telecommunications industry. As the demand for high-speed access to data networks, including both the Internet and private networks, continues to evolve, network managers face an increasing need to transport data signals over short distances. For example, in corporate campus environments, it is often necessary to implement high-speed network connections between buildings rapidly and inexpensively, without incurring commitments for long-term service contracts with local telephone companies. Other needs occur in residential areas, including apartment buildings, and even private suburban neighborhoods. Each of these settings requires efficient distribution of high-speed data signals to a number of locations.
An emerging class of products provides a broadband wireless access solution via point-to-point communication links over radio carrier frequencies in the microwave radio band. The telecommunications transport signals may be provided on a wire, but increasingly, these are provided on optical fiber media. An optical to electrical conversion stage is thus first required to convert the baseband digital signal. Next, a microwave frequency radio is needed to up-convert the broadband digital signal to a suitable radio carrier frequency. These up-converters are typically implemented using multi-stage heterodyne receivers and transmitters such that the input baseband signal is modulated and then up-converted to the desired radio frequency. For example, in the of an OC-3 rate optical transport signal having a bandwidth of 155 MegaHertz (MHz), the input signal may be up converted to an ultimate microwave carrier of, for example, 23 GHz, through several Intermediate Frequency (IF) stages at lower radio frequencies.
Other implementations may use optical technologies to transport the signal over the air. These technologies use optical emitters and detectors operating in the high infrared range. While this approach avoids conversion of the optical input to an electrical signal, it has certain limitations. First, the light wave carrier has a narrow beamwidth, meaning that the transmitter and receiver must be carefully aligned with one another. Light wave carriers are also more susceptible to changes in physical conditions. These changes may be a result of changes in sunlight and shade exposure, or foreign material causing the lenses to become dirty over time. Other problems may occur due to vibrations from nearby passing automobiles and heating ventilating and cooling equipment. Some members of the public are concerned with possible eye damage from high powered lasers.
The present invention is a point-to-point microwave radio link that operates in a Frequency Division Duplex (FDD) mode using separate microwave band radio frequency carriers for each direction. The transmitter uses direct digital modulation to convert an input baseband optical rate signal to the desired microwave frequency carrier. The design may be targeted for operation at unallocated frequencies in the millimeter wave spectrum, such from 40-320 GHz.
The direct digital modulation mechanism is implemented using a Continuous Phase-Frequency Shift Keyed (CP-FSK) scheme. The CP-FSK signal is generated at the transmitter by a circuit that uses a stable voltage controlled oscillator operating in the 10-13 GHz band. The VCO is deviated over a narrow frequency range, such as 10-20 MHz. The narrow deviation range need only be a fraction of the ultimately desired deviation range of the microwave carrier, because of the use of a frequency multiplier. In particular, the VCO output is fed to a frequency multiplier that multiplies the modulated microwave signal output to a higher output carrier frequency. A bandpass filter and power amplifier then feed a final stage filter and antenna.
The deviation frequency of the CP-FM modulator is thus chosen to be the reciprocal of the multiplication factor implemented by the frequency multiplier times the desired bit rate. For example, where it is desired to generate an output microwave signal in the 48-52 GHz range for a OC-3 input optical signal, the frequency multiplier may multiply the oscillator output by a factor of four. In this instance the frequency deviation chosen for the direct digital modulator is therefore equal to the input data rate divided by four. In the case of an input OC-3 rate digital data signal, the input data rate is 155.22 Megabits per second (Mbps), meaning that the required VCO deviation is therefore 38.88 MHz. In a case where a frequency multiplication factor of eight is introduced in the output signal processing chain, the VCO deviation may be further reduced accordingly.
The receiver uses a similar but inverse signal chain consisting of a microwave oscillator, frequency multiplier, and bandpass filter. A single down conversion stage is all that is required. By inserting the frequency multiplier between the oscillator and down convertor mixer, the local oscillator remains offset by a wide margin from the input RF carrier frequency. This permits the receiver image reject filters to be implemented more easily.
While the direct digital modulation approach is not necessarily bandwidth-efficient, it provides a low cost alternative to traditional approaches, since the base band modem and multiple RF stages are eliminated. Because there are no heterodyne stages, there also are no images of the modulated baseband signals created on either side of the carrier frequency. Thus, image reject filters are not necessary.
Direct digital modulation also only creates modulation artifacts at high multiples of the VCO center frequency. This allows the output bandpass filters to be implemented using inexpensive waveguide technologies that can easily reject the harmonics of the VCO output, as opposed to more stringent filters that might otherwise be required to reject the harmonics of the baseband signal.